Remove :$CWD param in IO::Path.dir
Remove :$CWD param in IO::Path.dir

Fixes RT#132675: https://rt.perl.org/Ticket/Display.html?id=132675

It was never documented and not specced in any released language.
We already have perfectly usable $!CWD for the purpose and the
bug in the ticket was caused precisely by the confusion having
two CWD knobs to tweak brings.

For &dir, We already have two ways to change CWD: $*CWD that'd affect
how given Str arg would be IO::Path-ified as well as $.CWD
attribute on IO::Path objects themselves.

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